1. Field of the Invention
The invention relates to water-soluble (or water-dispersible polyesters suitable as emulsifiers for alkyd resins, to the high-solids alkyd resin emulsions prepared therewith, and to their use.
2. Description of the Prior Art
In recent years, water-dilutable binders have been developed for paints and other coating compositions in various forms, essentially for environmental reasons. For reasons of preparation, stabilization or their further processing, the aqueous alkyd resin emulsions used for high-quality coating compositions have a low mass fraction of solids of not more than 45%, and they also usually comprise organic solvents and/or volatile neutralizing agents, such as amines and ammonia.
It is true that the amount of organic solvents may be reduced, as described in U.S. Pat. No. 3,269,967 and 3,440,193, using emulsifiers such as alkylphenol ethoxylates and fatty alcohol ethoxylates. These low molar mass substances remain unchanged in the cured film, however, and influence the service properties of the films adversely, such as the rate and extent of drying, hardness, water resistance, and weathering stability.
In the Austrian patents AT-B 325 742, 333 035 and 336 277, polyethylene-glycol-modified emulsifier resins are used to emulsify alkyd resins, as a result of which the aforementioned disadvantages are said to be substantially avoided. However, it is necessary to add a large amount of neutralizing agents and organic cosolvents to these binders in order to obtain stable emulsions. Owing to the high amount of emulsifier resin which is necessary in this case, the films obtained from such binder emulsions likewise exhibit deficiencies in water resistance and weathering stability.
Using both alkoxypolyethylene glycol (polyethylene glycol monoalkyl ethers) and cocondensed units derived from polyhydroxycarboxylic acids to hydrophilicise the alkyd resin does lead, according to U.S. Pat. No. 3,457,206, in conjunction with the neutralization of free carboxyl groups of the resin using alkanolamines and alkali metal hydroxides, to aqueous alkyd resins, but these resins are unsuitable for high-solids coating compositions and exhibit poor weathering stability.
According to WO 94/18260, the use of alkoxypolyethylene glycol makes it possible, even without the neutralization of acidic groups, to obtain water-soluble resins which find application as dispersants or wood preservatives. These resins, however, are readily hydrolysable in aqueous solution and cannot be diluted with water until shortly before use, and they are unsuitable for weather-resistant coatings.
Surprisingly it has now been found that high-solids aqueous emulsions of alkyd resins are obtained if the emulsions are prepared using selected polyesters as emulsifier resins and under certain process conditions. The term xe2x80x9chigh-solidsxe2x80x9d here refers to aqueous alkyd resin emulsions having a mass fraction of solids of 55% or more, preferably at least 60%.
The present invention accordingly provides a water-soluble or water-dispersible polyester B as emulsifier for aqueous emulsions of alkyd resins A, wherein the polyester B is obtained by reacting, under polycondensation conditions, polyhydroxyl components Ba from the class of the sugar alcohols having more than 4 carbon atoms and at least three, preferably at least four, and with particular preference at least five hydroxyl groups per molecule, alkoxypolyethylene glycols Bb having 1 to 4 carbon atoms in the alkyl radical and a number-average molar mass Mn of from 500 to 2000 g/mol, cycloaliphatic dicarboxylic acids Bc and unsaturated or saturated fatty acids Bd having 10 to 22 carbon atoms.
Monocarboxylic acids Be which are different from the fatty acids Bd may also be used as further components in the mixture of starting materials.
To prepare the polyesters B it is preferred to use the following mass fractions in the mixture of starting materials:
from 10 to 40%, preferably from 15 to 35%, of the polyhydroxyl component Ba,
from 35 to 70%, preferably from 40 to 60%, of the alkoxypolyethylene glycol Bb,
from 10 to 30%, preferably from 15 to 25%, of the cycloaliphatic dicarboxylic acid Bc,
from 15 to 40%, preferably from 20 to 35%, of the fatty acid Bd, and
from 0 to 15%, preferably from 3 to 10%, of another monocarboxylic acid Be which is different from Bd.
The polycondensation is conducted preferably up to an acid number of the polycondensate B of less than 10 mg/g, the solution of B in water having a mass fraction of solids of from 30 to 60% and a dynamic viscosity of from 5000 to 50,000 mPaxc2x7s.
The invention further provides aqueous alkyd resin emulsions having a mass fraction of solids of preferably from 60 to 80%, comprising mass fractions, based on the solids of the emulsion, of from 70 to 97%, preferably from 80 to 95%, of a water-insoluble alkyd resin A having an oil content of from 40 to 85%, preferably from 45 to 75%, and an acid number of less than 15 mg/g, preferably less than 10 mg/g, and from 3 to 30%, preferably from 5 to 20%, of the polyester B used as emulsifying resin. The mass fractions of A and B are chosen so that their sum is 100%.
The present invention still further provides a process for preparing the aqueous alkyd resin emulsion of the invention, comprising the synthesis of the emulsifier resin component B by reacting the stated components Ba to Be at a temperature of from 160 to 250xc2x0 C. under esterification conditions, it being possible also to add the conventional amounts of the known catalysts which accelerate the esterification reaction.
The water formed by the esterification reaction is preferably stripped off by means of an inert gas stream which is passed through the reaction mixture or by means of an organic solvent which forms an azeotrope with water. In the latter case, following separation of the water, the solvent is recycled to the reaction mixture and distilled off after the end of the reaction. After this come the following steps: diluting the resultant emulsifier resin with water to a mass fraction of solids of from about 30 to about 60%, the viscosity of the aqueous solution being preferably between 5000 and 50,000 mPaxc2x7s; mixing the water-insoluble alkyd resin A with the aqueous solution of the emulsifier resin at a temperature below 100xc2x0 C.; and, finally, diluting the resultant mixture with water to a mass fraction of solids of the resultant emulsion of preferably from about 60 to 80% and a dynamic viscosity of preferably from 500 to 3000 mPaxc2x7s.
The mixing of alkyd resin A and emulsifier B may take place such that the alkyd resin in liquid or, if desired, melted form is stirred into the aqueous solution of the polyester. It is also possible to stir the aqueous solution of the polyester B into the liquid or melted alkyd resin, in which case its temperature should then preferably not be more than 120xc2x0 C. One advantageous embodiment is the preparation of the emulsion in a mixing tube which preferably contains static mixers. A further favorable mode of preparation is that of mixing in a rotor/stator dispersing unit. In each case, however, it is necessary to prepare an aqueous solution or dispersion of the polyester B beforehand.
The invention still further provides for the use of the alkyd resin emulsions of the invention as binders in coating compositions, especially in paints and other coating materials.
It was surprising and unforeseeable that by virtue of the specific structure of the water-soluble polyester B used as emulsifying resin a sufficiently high hydrolysis stability of the aqueous solution and a broad compatibility with the alkyd resins to be emulsified are achieved which make it possible to emulsify solvent-free alkyd resins in the form of their melt at a temperature of up to 100xc2x0 C.
During the mixing of the components, an oil-in-water emulsion is formed, so that the emulsion obtained may be diluted to the desired solids content after just short homogenization. As a result, there is no incidence of the very sharp increases in viscosity which are found at the beginning of the addition of water in conventional processes, in which the emulsifier or emulsifier resin is premixed with the resin to be emulsified, and which may prevent or at least hinder homogenization.
With the aid of the water-soluble polyesters B of the invention and of the process described, it is possible to convert different alkyd resins A, which per se are not water-soluble and which serve as binders for coating materials, into stable aqueous emulsions suitable for further processing and application. The preferred water-insoluble alkyd resins include medium-oil or long-oil alkyd resins having an oil content (mass fraction) of from 40 to 85%, which may if desired have been modified in a known manner, for example, by means of rosin or the incorporation of urethane groups. Mixtures of these resins may of course also be employed, provided they are xe2x80x9ccompatiblexe2x80x9d (i.e., miscible without phase separation) with one another.
The oil content is defined as the mass fraction of the oils, or of the units derived from oils, in the alkyd resin, always calculated as triglycerides.
The acid number is defined in accordance with DIN 53 402 (DIN EN ISO 3682) as the ratio of the mass mKOH of potassium hydroxide required to neutralise a sample under analysis to the mass mB of this sample (mass of the solids in the sample in the case of solutions or dispersions); its customary unit is xe2x80x9cmg/gxe2x80x9d.
The structure and the preparation of these resins A are known to the skilled worker and described at length in the relevant technical literature. The resins are used advantageously in their 100% form (i.e., without the addition of solvents or diluents).
The water-soluble polyester used as component B serves as an emulsifying resin and is used in the form of its aqueous solution having a mass fraction of solids of from 30 to 60% and a dynamic viscosity of from 5000 to 50,000 mPaxc2x7s. The pH of this solution is from 2.5 to 5 but has little influence on the stability of the products of the invention. Thus, these solutions are notable for a constant viscosity and minimal increase in acid number during a storage period of 4 weeks. The hydrolysis-resistant character is apparently achieved by the use, in accordance with the invention, of sugar alcohols as polyhydroxyl component and cycloaliphatic dicarboxylic acids as polycarboxyl component.
In the esterification of the sugar alcohols Ba there is probably dehydration and ring closure to give anhydro forms, thereby surprisingly achieving better compatibility with the alkyd resins to be emulsified and higher stability of the alkyd resin emulsions of the invention.
Sugar alcohols Ba used are preferably hexitols (C6 sugar alcohols), but they may be partly or even wholly replaced by pentitols (C5 sugar alcohols). Among the hexitols, preference is given to the use of sorbitol, mannitol and dulcitol, and among the pentitols, to the use of arabitol and xylitol, and also mixtures thereof.
The alkoxypolyethylene glycols Bb have 1 to 8, preferably 1 to 4, carbon atoms in the alkoxy radical and a number-average molar mass of from 500 to 2000, preferably from 600 to 1800, g/mol. Up to 20% of the oxyethylene units may also have been replaced by oxypropylene units. Particular preference is given to methoxy-, ethoxy- and n-butoxy-polyethylene glycols having a number-average molar mass of from 600 to 1800 g/mol.
Suitable cycloaliphatic dicarboxylic acids Bc are saturated and also partly unsaturated acids having 6 to 12, especially 8 to 10, carbon atoms, such as hexahydrophthalic acid, tetrahydrophthalic acid and its alkyl derivatives, e.g., methyltetrahydrophthalic acid, which are used preferably in the form of their anhydrides. It is likewise possible to use cyclohexane-1,3- and -1,4-dicarboxylic acid and the dicarboxylic acids derived from polycyclic, especially bicyclic hydrocarbons, or the anhydrides of these acids, such as; for example, bicyclo[2.2.2]oct-5-ene-2,3-dicarboxylic anhydride. Surprisingly, fumaric acid, succinic anhydride and maleic anhydride, and also aromatic dicarboxylic acids such as phthalic acid, isophthalic acid and terephthalic acid, for example, have not proven suitable; they result in a higher hydrolysis rate and poorer stability of the alkyd resin emulsions.
For the emulsification of oxidatively drying alkyd resins, preference is given to unsaturated fatty acids Bd having an iodine number of at least 100, preferably from 120 to 200, in which isolated and conjugated double bonds may be present. They are obtained, for example, from vegetable oils such as soybean oil, from fish oil, from sunflower oil, linseed oil, safflower oil, and cottonseed oil by hydrolysis, or originate from the distillation of tall oil. Fatty acids with conjugated double bonds .are obtained by catalytic isomerization of natural fatty acids or from dehydrated castor oil (ricinene fatty acid).
In addition, it is also possible, in whole or in part, to use saturated fatty acids Bd having 10 to 22 carbon atoms, provided oxidative drying of component B is not required. Suitable examples include lauric acid,-palmitic acid, and stearic acid.
In order to modify component B it is also possible to use other monocarboxylic acids Be having 6 to 40 carbon atoms which are not fatty acids, especially branched or cyclic aliphatic or else aromatic carboxylic acids. Examples of suitable acids are benzoic acid, tert-butylbenzoic acid, resin acids as present in rosin, and branched carboxylic acids such as isononanoic acid or Versatic acid, for example.
To prepare the emulsion, the components are mixed in a mass ratio of preferably from 70 to 95% of component A and from 5 to 30% of component B (in the form of the aqueous solution), based in each case on the mass of the solids. After thorough homogenization, the emulsion obtained in this way may be diluted to the desired solids content, exhibiting a uniform drop in viscosity if further amounts of water are added.
In the process of the invention, melted solid resins having a temperature of up to 120xc2x0 C. may also be added as component A to the cold, aqueous emulsifier resin solution, so as to give a mixing temperature of not more than 100xc2x0 C. The addition of the aqueous emulsifying resin solution to the melted solid resin is also possible, provided a mixing temperature of 100xc2x0 C. is not exceeded as a result.
Depending on the nature of the emulsified alkyd resins the emulsions prepared in accordance with the invention may be used as binders for paints, and also for coating compositions of textiles or mineral materials and the like. In the coatings sector, it is possible to formulate clearcoats, glazes, primers and topcoats on the basis of the emulsions of the invention.
For the coating of workpieces comprising iron or steel, the emulsions are preferably used with a pH of more than 7, in order to avoid the formation of flash rust which occurs when acidic aqueous paint systems are used. The establishment of the pH using alkali metal hydroxides or alkaline earth metal hydroxides may take place in any phase of the process, having no influence on the stability of the emulsion.